The basic functional of the auditory system is to allow listeners to detect signals in quiet as well as noisy environments. The ability to detect sounds in noisy environments is compromised in the elderly and the hearing impaired. The representation of signals in noisy environments in the brain is enhanced by feedback connections, which are not fully functional in anesthetized and decerebrate preparations but are in the awake and behaving condition. The goal of this project is to study the representation of signals in quiet and noisy environments in awake and behaving primates. Primates are the choice for this study due to their phylogenetic similarity with humans, as well as the similarity of the primate and human brainstem, which are different from other mammals. Previous studies in cats have shown that the representation of signals in noise can be correlated with receptive field organization and response properties. If sustained noise causes degradation of signal representation, then rapid adaptation to noise would enhance representation, and basic response properties used to characterize a unit may be a good predictor of signal representation quality in noise. This project proposes to test these hypotheses by recoding single unit responses in the cochlear nucleus and inferior colliculus in awake and behaving primates, whose efferent systems are intact and functional. Overall, this study will help determine some of the mechanisms that preserve our ability to hear in high levels of noise.